1. Field of the Invention
The present invention relates to an apparatus for producing reduced iron, which comprises mixing a reducing agent and iron oxide, agglomerating the mixture, drying the compacts (pellets, or briquettes) in a dryer, and reducing the dried compacts in a high temperature atmosphere in a reducing furnace. This invention also relates to a method for drying compacts, which method is applied to the apparatus.
2. Description of the Related Art
To produce reduced iron, the first step is to mix an iron ore powder, a coal powder, a limestone powder, and a binder, and compress and agglomerate the mixture to form wet compacts called green compacts. Then, the wet compacts are dried to some degree to form dry compacts. The dry compacts are heated to a high temperature in a reducing furnace, where iron oxide in the iron ore is reduced with the coal (a reducing agent) to form reduced iron compacts.
An example of a conventional apparatus for producing reduced iron is explained by way of FIG. 6. At an upper portion of a circular rotary hearth type reducing furnace 1, there are provided a device 2 for introducing compacts (dry compacts), and an off-gas duct 3 for discharging a hot off-gas, a residual form of a gas used in reduction. Inside the furnace 1, a discharger 4 is provided for discharging reduced compacts P (reduced iron compacts). On a circumferential side wall of the furnace 1, a plurality of burners 5 are provided for generating a reducing hot gas.
Powders of coal (a reducing agent), iron ore, etc. as raw materials are mixed with a binder, and the mixture is fed to a pelletizer or a briquetter 6, where compacts (wet compacts) are formed. The resulting compacts are sent to a dryer 7, where the compacts are dried at about 120 to 150xc2x0 C. to become dry compacts. The dry compacts are supplied to a rotary hearth of the reducing furnace 1 via the introducing device 2.
In the reducing furnace 1, fuel and combustion air are fed to the burners 5, which generate a high temperature hot gas. The hot gas turns in the direction of a dashed arrow, and during this motion, exerts a reductive action on the compacts, an object to be treated, in a high temperature atmosphere. A hot off-gas discharged through the off-gas duct 3 is primarily cooled by a primary cooler 8 of a water spray type, and then brought to a heat exchanger 9, where the off-gas exchanges heat with the combustion air. Further, the off-gas is secondarily cooled by a secondary cooler 10 of a water spray type to about 300xc2x0 C., for example. Then, the cooled off-gas is conveyed to the dryer 7 to dry the compacts. Then, the off-gas is passed through a dust collector 11, where it is cleaned, and then dissipated into the air.
When the rotary hearth inside the reducing furnace 1 makes nearly one rotation in the direction of a solid arrow in FIG. 6, the reduced compacts P are discharged from the screw type discharger 4. The compacts are delivered to a portable container 13 by a discharge chute 12, and then transported to a subsequent step.
In the dryer 7 of the foregoing conventional apparatus for producing reduced iron, the hot off-gas discharged from the reducing furnace 1 and cooled by the water spray type primary cooler 8 and secondary cooler 10 is used as a heat source for drying the compacts (wet compacts) at a temperature of about 120 to 150xc2x0 C. That is, the compacts are dried with the hot off-gas very rich in steam. Hence, on an unsteady occasion immediately after initiation of operation of the apparatus, moisture is condensed onto the surfaces of the wet compacts. As a result, sticking of the wet compacts to each other occurs, whereupon the wet compacts may lump, becoming large masses. In a situation such as that immediately after start of operation, the properties, such as temperature and flow rate, of the hot off-gas discharged from the reducing furnace 1 are not stable, so that drying in the dryer 7 is unstable. This may cause the problem of wet compact lumping.
The wet compacts, which have been treated in the dryer 7 during such an unsteady operation, may have moisture remaining in the compacts. If such wet compacts are rapidly heated in the reducing furnace 1 in a subsequent step, surface portions of the compacts may peel off, or the compacts may rupture.
A heating gas in a dryer like the above-mentioned dryer 7, or in a dryer using hot air from a heat exchanger or the like as a heat source for drying compacts (wet compacts), may cause compact rupture, or formation of a combustible gas from coal in the compacts, if the temperature of the heating gas is high. To avoid these risks, the maximum temperature of the heating gas is set at 200xc2x0 C. or lower. Depending on the moisture content, etc. of the heating gas, however, a higher gas temperature than that may be set. Since a conventional dryer uses a heating gas whose temperature has been set to be somewhat low, it has posed the problem of taking time for drying compacts.
In an apparatus for producing reduced iron, which uses coal as a reducing agent, volatile matter (hereinafter referred to as VM) , such as CO, CH4, H2O, CO2 and N2, occurs from coal, if the temperature of the heating gas is too high. At a high oxygen concentration, therefore, coal may catch fire. Once VM develops in the dryer, the VM cannot be utilized as a heat source in the reducing furnace in the subsequent step. This poses the disadvantage that the thermal efficiency of the reducing furnace lowers. If the temperature of the heating gas is the sulfuric acid dew point (120xc2x0 C.) or lower, on the other hand, corrosion will be induced because of dew formation in piping, etc. inside the dryer.
As described above, temperature control for the heating gas is of vital importance in efficiently drying compacts (wet compacts) in a dryer. There has been an intense demand for the realization of a dryer capable of stable drying.
The present invention has been proposed in light of these circumstances. It is an object of this invention to provide an apparatus for producing reduced iron, which can perform highly efficient, stable drying in a dryer and produce high quality reduced iron stably; and also to provide a method for drying compacts which is applied to the apparatus.
A first aspect of the present invention, as a means of attaining the above object, is a method for drying compacts, the method being applied to an apparatus for producing reduced iron by mixing and agglomerating a powder of a reducing agent and a powder of iron oxide in a pelletizer to form compacts or in a briquetter to form briquettes, drying the compacts (pellets, or briquettes) in a dryer, and reducing the dried compacts in a high temperature atmosphere in a reducing furnace, wherein
a temperature range of a heating gas supplied to the dryer is set based on the following equation:
Sulfuric acid dew pointxe2x89xa6Tgxe2x89xa6100/40xc2x7CH2O+200
where Tg denotes the temperature [xc2x0 C.] of the heating gas, and CH2O denotes a moisture concentration [vol %] in the heating gas.
According to the above aspect of the invention, a high gas temperature adapted for the moisture concentration (moisture content) in the heating gas can be set. Thus, the drying time can be shortened, and highly efficient, stable drying can be performed, so that high quality reduced iron can be produced stably. Furthermore, the temperature of the heating gas on the exit side of the dryer is a high temperature above the sulfuric acid dew point. Thus, acid corrosion of piping, etc. minimally occurs.
In the method for drying compacts as the first aspect of the invention, the apparatus for producing reduced iron may use coal as the reducing agent, and the temperature Tg of the heating gas may be set at Tgxe2x89xa6300xc2x0 C. Thus, compact rupture or formation of VM from coal in the dryer can be prevented. Consequently, ignition of coal, or a decrease in the thermal efficiency in the reducing furnace in the subsequent step can be prevented.
A second aspect of the invention is an apparatus for producing reduced iron, comprising a pelletizer or a briquetter for mixing and agglomerating a reducing agent and iron oxide to form compacts, a dryer for drying the compacts, a reducing furnace for reducing the dried compacts in a high temperature atmosphere, a first heat exchanger for performing heat exchange between a hot off-gas discharged from the reducing furnace and combustion air to be supplied to the reducing furnace, and a cooler for cooling the hot off-gas, wherein
a second heat exchanger for heating drying air is disposed on an exit side of the first heat exchanger, and the drying air heated by the second heat exchanger is supplied to the dryer.
According to this aspect of the invention, compacts (wet compacts) are dried with moisture-poor drying air. Thus, sticking of the compacts to each other does not take place (compacts are prevented from becoming large lumps), and the compacts are uniformly dried. Since the compacts are uniformly dried to leave no moisture behind inside the compacts, peeling of the surface portion, or rupture of the compacts can be avoided in the reducing furnace in the subsequent step. Moreover, high quality dry compacts are formed, and the supply of these compacts to the rotary hearth type reducing furnace can result in the stable production of high quality reduced iron.
In the apparatus for producing reduced iron as the second aspect of the invention, the cooler may be a water spray type first cooler provided upstream from the first heat exchanger, an air introduction type second cooler may be provided on a path bypassing the first cooler, and a control means may be provided for switching a valve provided at a bifurcation upstream from the bypass path to select either the first cooler or the second cooler based on a trade-off between a flow rate of the hot off-gas and a flow rate of the compacts to the dryer. Thus, in addition to the same actions and effects as obtained by the second aspect of the invention, the heat exchange efficiency of the first heat exchanger and the second heat exchanger is increased. Consequently, even more efficient, stable drying can be performed in the dryer.
In the apparatus for producing reduced iron as the second aspect of the invention, moreover, a hot stove for generating a hot gas may be disposed on a drying air introduction side of the dryer, and the hot gas generated by the hot stove and the drying air heated by the second heat exchanger may be supplied to the dryer. Thus, in addition to the same actions and effects as obtained by the second aspect of the invention, there is obtained the advantage that the temperature and flow rate of the drying air fed to the dryer can be adjusted more easily.
A third aspect of the present invention is an apparatus for producing reduced iron by agglomerating a powder of a reducing agent and a powder of iron oxide in a pelletizer to form compacts or in a briquetter to form briquettes, drying the compacts in a dryer, and reducing the dried compacts in a high temperature atmosphere in a reducing furnace, wherein
a hot stove for generating a hot gas is disposed on a drying gas introduction side of the dryer, and the hot gas from the hot stove is supplied to the dryer as a drying gas.
According to this aspect of the invention, in addition to the same actions and effects as obtained by the second aspect of the invention, there is obtained the advantage that the operation of the dryer can be controlled easily by arbitrarily adjusting the temperature and flow rate of the hot gas generated by the hot stove.